Webmasters who patched their sites against a serious SSL flaw discovered in October will have to check them again. Researchers have discovered that the vulnerability also affects implementations of the newer TLS (Transport Layer Security) protocol.

The POODLE (Padding Oracle On Downgraded Legacy Encryption) vulnerability allows attackers who manage to intercept traffic between a user's browser and an HTTPS (HTTP Secure) website to decrypt sensitive information, like the user's authentication cookies.

Initially, researchers believed it affected only SSL 3.0, an aging protocol superseded by TLS 1.0, 1.1. and 1.2. That still put users at risk, since most browsers and servers still supported SSL 3.0 for backward-compatibility reasons. Attackers were able to force a connection downgrade from TLS to SSL and then exploit the vulnerability.

Security researchers have now discovered that the issue also affects some implementations of TLS in products that don't properly check the structure of the "padding" used in TLS packets.

The problem was first observed in old versions of Mozilla's NSS (Network Security Services), the cryptographic library used in Firefox and other products, but Google security engineer Adam Langley built a scanner to find out if other products are affected.

He found that some major sites were vulnerable, and it turned out to be because they were using load balancers from F5 Networks and A10 Networks to handle the TLS connections.

"F5 have posted patches for their products and A10 should be releasing updates today," Langley said Monday in a blog post. "I'm not completely sure that I've found every affected vendor but, now that this issue is public, any other affected products should quickly come to light."

According to Ivan Ristic, who runs the SSL Labs at security vendor Qualys, about 10 percent of servers monitored by the SSL Pulse project are vulnerable to POODLE attacks through TLS. The SSL Pulse project monitors the HTTPS-enabled sites from the list of top 1 million most visited sites published by Internet statistics firm Alexa -- around 151,000 sites in November.

Website administrators who want to check if their servers -- or load balancers used in front of their servers -- are vulnerable, can use the Qualys SSL Labs server test, which has been updated to detect the problem.

"If vulnerable, apply the patch provided by your vendor," Ivan Ristic said in a blog post. "As problems go, this one should be easy to fix."